The invention relates to a method of welding two generally rod-shaped structures of a fluorine-containing plastic material in a butt weld comprising the steps of:
placing the end faces of the structures in a contacting relationship by supporting each structure in a clamping device at a distance from its end face, and
applying heat to the end parts of the structures that project from the clamping device, and
pressing the end faces together by moving the clamping devices towards each other such that a welding pressure of between 0.01 and 0.5 Nmmxe2x88x922,  perpendicular to the end faces, is transmitted from the clamping devices to the end faces via the end parts of the structures, wherein the distance of the heating element from the end parts is sufficient to substantially prevent the heated parts of the structures from contacting the heating element.
The invention also relates to a method for replacing and repairing a sealing element of a swivel.
In the offshore technology, subsea hydrocarbon structures such as oil- or gas wells, are connected to floating production or processing vessels via one or more risers extending from the bottom of the sea to the vessel. The risers are connected to a piping structure on a turret around which the production vessel can weathervane to adjust its position according to prevailing wind and current directions. The geostationary parts of the product risers are connected to a stationary wall (for instance the inner wall) of respective swivel on the turret. The swivels are generally arranged in a swivel stack. Product piping for distributing the hydrocarbons to the weathervaning part of the vessel are connected to for instance the outer annular wall of the swivel which can rotate with the weathervaning vessel around the fixed-position inner wall. The rotating inner and outer walls are sealed by circular polytetrafluoroethylene (PTFE) sealing elements placed in a seal cavity and extending in the pressure extrusion gaps between the walls. The seal cavity may be formed in the inner swivel wall, the outer swivel wall, or both walls. The sealing element may be a piston type seal, a face type seal or a reinforcement ring and should be able to withstand pressures amounting up to 500 bar. The diameter of the seals may vary between for instance 20 cm and 3 m. In the extrusion gap, reinforcement rings may be placed against the seal, which are ring-shaped elements with a square cross-section, whereas the seal itself generally has a U- or V-shaped cross-section.
When a seal or reinforcement ring has to be replaced, the swivel stack needs to be dismantled. The inner and outer piping has to be detached from the inner and outer swivel walls and the non-functional swivel is radially taken out of the stack. Thereafter the inner and outer walls are separated, the sealing element is taken out and is replaced by a closed loop functional sealing element. Thereafter the annular walls are reassembled and the swivel is placed back in its swivel stack, whereafter the product piping is reconnected. This leads to a very long downtime and consequently high costs of the swivel seal repair. It is also known to add additional backup seals on a swivel stack upon installation for replacing a defective seal. By the use of extra backup seals already placed around the inner product piping, the non-functional seal can be changed out quickly in an emergency case without the need for removal of the inner piping and dismantling of the complete swivel stack. However, it is not possible to place additional backup seals around existing swivels in an easy manner.
From U.S. Pat. No. 3,946,136 it is known to connect two objects of polytetrafluoroethylene (PTFE) using an interlayer between two surfaces that are to be connected, the interlayer comprising between 90% and 99.5% by weight of tetrafluoroethylene and 0.5% to 10% by weight of perfluoro-alkyl-perfluorovinyl. A process is described wherein the two PTFE-layers are placed in an overlapping relationship between horizontal plates and are heated up to a temperature of 400xc2x0 C. while exerting a clamping pressure in the range of from 0,1 to 20 kilogram per cm2 for a time of from 1 to 60 minutes.
From U.S. Pat. No. 4,211,594 a method is known for forming a butt joint between two abutting surfaces of structures formed of PTFE, According to this method, beating elements form a chamber around the butt seal wherein the heated end parts of the structures that are joined can expand and contact the heating element upon heating. In this way sufficient sealing pressure is provided on the end parts such as between 60 and 300 N per cm2. Films of a fluorine-containing plastic material may be inserted into the gap between the abutting surfaces such as films of perfluoroalkoxypolymer (PFA). This method has as a disadvantage that distortions in the welded area are formed such that welds made according to this method are not acceptable, for instance for forming high pressure swivel seals of a fluorine containing thermoplastic material.
From U.S. Pat. No. 4,197,149 a method for forming a butt joint is known. According to this method, the heating element is spaced at a distance from the weld area. The clamping elements are provided with cooling channels for removal of heat from the weld area. With the known method, however, an accurate temperature control of the weld area is difficult. For some applications, such as for forming high pressure swivel seals, an accurate control of the temperature of the weld area is critical for obtaining a high quality weld.
It is an object of the present invention to provide a method of welding two generally rod-shaped structures of a fluorine-containing thermoplastic material in a butt weld by which the temperature of the weld area can be accurately controlled and which gives a strong seam with a smooth outer surface. It is a further object of the present invention to provide a method of welding two rod-shaped structures for forming a large diameter high pressure swivel seal. It is again another object of the present invention to provide a method of forming or repairing high pressure swivel seals which is relatively simple and which reduces down time upon repair.
Hereto the method according to the present invention is characterised in that the rod-shaped structures are welded to form a closed-loop structure, wherein the heating element surrounds the end parts along a substantially closed contour.
By using the end parts of the structures that are able to transmit the welding pressures due to their structural stiffness and shape-stability, it is not necessary to contact the sealing area with the heating element. In this way a very uniform and non-deformed butt weld can be produced. Because the heating element does not directly contact the end parts, a very uniform temperature distribution throughout the sealing volume can be achieved without degradation of the fluorine containing plastic material. Because the heating element substantially surrounds the end parts, a very accurate temperature control of all parts of the weld can be achieved, resulting in a very uniform, strong and reliable weld.
The heating element may be constructed upon welding from different separate parts which are connected to surround the closed-loop of fluorine-containing plastic material. After completion of the weld, the heating element may be disassembled. It is however preferred for quick operation, especially under offshore conditions, that the heating element is of a generally ring-shaped construction comprising two segments which are moved apart during placement and/or removal of the heating elements around the end parts, which segments are moved towards one another for carrying out the welding. The heating element can be quickly put in place by hinging the two segments apart along a flexible part thereof or alone a hingeline. After introduction of the end parts which are to be welded, the ring-segments of the heating element can be hinged together such that the heating element is closed such that a very uniform heating of the weld area can be obtained. After completion of welding the closed-loop construction, the heating element is opened and removed from the closed-loop construction.
In one embodiment, the heating element comprises a heat generating member and a heat reservoir member located in the vicinity of the weld area, wherein the heating element is energized in accordance with a predetermined heating profile, wherein the temperature of the weld area is measured and the heating operation of the heat generating member is increased or decreased to transmit and absorb heat from and into the heat reservoir member such that the temperature of the weld area is maintained within a predetermined margin around the heating profile.
By providing a heat reservoir member, which may for instance be formed by a ring-shaped metal body having a large heat capacity, heat can be applied to and removed directly from the weld area, without having to be conducted through the clamping members such as in U.S. Pat. No. 4,197,149. By accurately monitoring the temperature of the weld area and by adding or removing heat from the weld area by means of the temperature reservoir member, a very accurate temperature control is possible such that a desired temperature profile, which may include a heating up phase, a stabilisation phase and a cooling down phase, may be followed within 1xc2x0 C.
As used herein, the term xe2x80x9crod-shapedxe2x80x9d is intended to mean end parts of a generally cylindrical, square, polygonal or any other cross-sectional shape in contrast to a generally two-dimensional sheet material. The fluorine containing thermoplastic material may comprise PTFE, PFA (which is considered as PTFE material with approximately 5 weight % PFA) or modified PTFE (which is considered as PTFE with less than 1% PFA) or any other fluorine containing thermoplastic material.
Preferably the heating element heats the end parts to a temperature between 300xc2x0 C. and 400xc2x0 C., preferably between 360xc2x0 C and 390xc2x0 C. The heating element surrounds the end parts along a substantially closed contour and can be formed of for instance an electrical resistance heater such as available from Exotherm Products LTD, South Glamorgan, UK. The distance between the heating element and the end parts may be between 0.5 mm and 10 mm.
In one embodiment of the method according to the present invention a welding material, such as for instance a PFA-material, is placed between the end faces prior to pressing the end faces together. Hereby the sealing temperatures and sealing pressures may be reduced and sealing may be achieved in for example 20 minutes. The welding material may comprise a film of a thickness between 0.01 mm and 1 mm. The use of a film allows for a small amount of surface misalignment and roughness to be tolerated during the welding process.
In one preferred embodiment of the method according to the present invention, the structures are formed by the end parts of a ring of uniform cross-section which may be a sealing ring or reinforcement ring for high pressure swivels having a diameter of between about 0.2 m and 3 m and a cross-sectional area of between about 1 cm2 and 4 cm2. Such sealing elements are used under pressures ranging from 50 to 500 bar and temperatures of up to 90xc2x0 C. The high pressure swivels of which the seals are part, comprise inner and outer annular walls which are rotatingly connected to each other for passing hydrocarbons from the inner part of the stationary inner wall to the rotating outer piping of the outer walls. Multiple swivels may be axially aligned in a swivel stack configuration. When a seal needs to be replaced upon failure, the swivel stack has to be disassembled and the swivel with the non-functional seal is completely removed after disconnecting and removing of the inner and outer piping. This results in a long period of shut down of the production process. The seal replacement or repair method according to the present invention is characterised in that the functional sealing element is in the form of an open loop of fluorine containing material, and is placed on the swivel either prior to or after removal of the non-functional sealing element by sealing the end parts in a butt weld to form a closed loop by the steps of:
placing each end part of the sealing element in a respective clamping device at a distance from an end face of each end part,
pressing the end faces together by moving the clamping devices towards each other such that a welding pressure perpendicular to the end faces is transmitted from the clamping devices to the end faces via the end parts of the sealing element, and
xe2x80x83heating the end faces by a heating element while maintaining the heating element at a sufficient distance from the end parts to substantially prevent the heated end parts of the sealing element from contacting the heating element, wherein the heating element surrounds the end parts along a substantially closed contour.
According to the present invention, a swivel with a damaged or non-functional seal no longer needs to be removed from the swivel stack. The sealing element according to the present invention can be exchanged while the inner and outer walls of the swivel are taken apart wherein the outer ends of the sealing element are sealed after putting the seal in its proper position by placing it in a radial direction around the inner annular wall, including the inner piping which remains attached during repair. This considerably reduces the down time upon swivel repair.